Effect of Wind Break Walls on Performance of a Cooling Tower Model

Transcription

1 Mech. & Aerospace Eng. J., Vol. 3, No. 4, Winter 2008, pp Effect of Wind Break Walls on Performance of a Cooling Tower Model J. Madad-Nia and H. Koosha M. Mirzaei Sydney Univ. of Tech., Aerospace Eng. Dep t. Australia K.N.Toosi Univ. of Tech. ABSTRACT Environmental cross winds usually distord the uniform distributions of both air flow and air resistance at inlet to cooling towers and subsequently cause hot areas due to insufficient local cooling of water by air. Curative devices, such as windbreaks, are developed to enhance the performance of the power station cooling towers. At present experimental investigation, the effects of curative devices on performance of a 1/1000 scaled isothermal model of a 660MW wet cooling tower were quantified, using intake dynamic and total pressure losses. Dimensional simulitudes were used in the isothermal modelling of the wet cooling tower. A dimensionless pressure loss coefficient in the tower was defined, as the performance indicator, which is the ratio of total pressure drop in the heat exchanger region (i.e. combined packing and rain zone) to the dynamic pressure. Over twenty three curative devices were designed, manufactured, and their effects on the pressure loss coefficient were quantified. Pressure drop coefficient with no curative device was selected as the base condition and was compared with the presence of curative devices. Up to 33% improvement was noticed in the pressure loss coefficient. The top three most efficient curative devices were selected and presented here. Key Words: Curative Device, Natural Draft Wet Cooling Towers, Windbreak Wall, Pressure Drop Coefficient ) () 1- Asistant Professor 2- Asistant Professor 3- Associate Professor (Corresponding Auther): mirzaei@kntu.ac.ir

2 62 Mech. & Aerospace Eng. J., Vol. 3, No. 4, Winter 2008 Nomenclature p t Dynamic Pressure Drop in the Model Tower (= H tower ) p w Dynamic Pressure Drop in the Wind Tunnel (= H wind ) p 0 Total Pressure Drop in the Tower (Measured at the Reference Positions Between Tower Outside and Inside = H 0 ) D Base Diameter of the Model Tower h Tower Intake Height a Circumference Gap Between 2 Break Walls b Length of a Break Wall c Radial Gap Between the Centre of Walls and the Outer Edge of the Model Tower d the Angle Between a Break Wall and Radial Direction e the Angle Between the Break Wall and it s Vertical Direction f Height of the Break Wall Air Density v tower Air Speed Inside the Tower U wind Air Speed in the Wind Tunnel at the Reference Location Cp the Dimensionless Tower Inlet Pressure Drop Coefficient Velocity Ratio ( V r Uwind / vtowe ) DCp Percentage of Performance Improvement ( DCp (Cp0 Cp) 100 ) Cp 0 Cp0 Pressure Loss Coefficient for the Case Without Device 1. Introduction Atmospheric boundary layers distort uniformity of air flow at inlet to the cooling towers, leading to lower performance and higher coolant wanter temperature from the cooling towers. The application of curative devices such as windbreak walls could ameliorate the performance of the cooling tower in decreasing the pressure drop coefficient [1-14]. The wind disturbs the natural induced flow and the use of the curative devices changes the air flow patern at the intake of the cooling tower which is more uniform flow at the intake. This work is part of a larger undertaking which is to be run as a joint venture between the University of Technology, Sydney (UTS), the University of New South Wales (UNSW), and Delta-Electricity which operates the 660 MWe Mount Piper thermal power station in New South Wales, Australia. Delta-Electricity collaborates with UTS and UNSW to design, develop and apply flow-conditioning devices on the cooling towers at the Mount-Piper power station. The research project involves combined studies on scaled and numerical models and full-scale prototype tower. This paper deals only with the design and testing of curative devices to determine their effects on the performance of a 1/1000 scale model cooling tower in a wind tunnel. The important factors in the model tests are tower inlet pressure loss, air velocity in the tower and wind velocity. Pressure tubes were used to measure the pressure distributions in the wind tunnel and inside the tower model. The pressure loss coefficient as a performance indicator was quantified and plotted against the velocity ratio.it is anticipated that the tower performance in windy conditions can be predicted from a plot of the experimentally determined tower inlet pressure loss coefficient. Curative devices were designed and installed on the model and their performance enhancement effects were quantified. The three most effective curative devices were selected and reported here. Zhenguo and Jinling [1] have observed the unfavorable effect of natural wind on dry cooling towers at a power plant in Shanxi Province. Measures to overcome such negative wind influence have been studied through model test in wind tunnel. Several arrangements have been investigated to improve the inlet airflow velocity distribution and thereby the tower cooling efficiency under natural wind conditions. They observed that the addition of 4 rectangular guide walls around the periphery of the tower air inlet could greatly improve the tower performance. Bender, Bergstrom and Rezkallah [2] have examined the use of wind walls placed upstream of the cooling tower to control the flow rate entering the intakes. A 1/25 scale model cooling tower was tested in the simulated atmospheric boundary layer of a wind tunnel to investigate the effect of different wind wall configurations. The results show that a simple wind wall placed upstream of the windward intake can be used to balance the flow rate into the intakes. The experimental results focused on the relative magnitude of the airflow rates. An optimal wall configuration was found by varying three wall parameters: wall placement, wall height and wall porosity. The best wind wall configuration based on the tests appears to be a 10% porous wall, with a rectangular shape and placed in front of the modeled cooling tower. The location of the wall and its porosity had the dominant effect on the cooling tower intake flow rate, whereas the wall height was less important. Du Preez [3] studied the effect of cross winds on the performance of natural draught dry-cooling towers by means of model tests, numerical simulations, and prototype measurement. He

3 Effect of Wind 63 developed a correlation for the tower pressure loss coefficient as a function of velocity ratio, diameter to intake height ratio, and pressure drop in the heat exchanger region. He noticed an increase in the tower pressure loss coefficient when the heat exchanger resistance decreased and argued that was due to a tendency for tower velocity distribution to become more non-uniform. He also highlighted that the tower performance generally becomes less sensitive to the wind for a reduction in the heat exchanger's pressure drop. For large values of the intake pressure, tower performances become independent of the pressure loss in the heat exchanger. Gandemer [4] examined a wide variety of wind breaks, including various wall geometries, double walls, walls with ramps and walls with directing fins. Gandemer s analysis was useful for determining the design of the walls adopted in the present study due to the number of characteristics taken into account. In a full scale measurements on the wet type natural draft cooling towers, Amur et el. [12], reported that for wind speed between 0 4 m/s, the tower approach temperature decreased to 3K when the wind was blowing from plant building sides. Contrary to that about 3K rise in the approach temperature was observed when the wind was blowing at similar speed from the second tower side. Amur et al. [13] also conducted experiment in a wind tunnel on a model tower. They reported similar results as the inlet pressure coefficient (Cpi) in the model tower was higher when the wind was blowing from the second tower side compared to that blowing from the plant building sides. By using a curative device made of several radial or angular walls, the tower Cpi improvements of up to 10% and 30% were observed for plant buildings and second tower sides respectively. Amur et al [14] also numerically modelled the wind effects on cooling towers and noticed that the tower performance was reduced when the wind is obstructed by another cooling tower. 2. Design of the Curative Devices A wide variety of curative devices were designed and tested in the wind tunnel. The three most effective curative devices were selected. The main selection criterion was improving the performance of the cooling tower without adverse effects in nowind conditions. Each device was positioned at the intake of the tower outside the rain zone. The following parameters as outlined in the Table 1 and Fig. 1 are considered in the performance analysis: Geometry of the curative device, dimensions such as a, b, c, d and e, porosity: porosity percentage, holes shape and holes slope, and wall material. The length of the break walls, their angle relative to the radial direction, and the gap between break walls were included in the performance studies. Wall geometry, the height of the break walls, the angle relative to the vertical direction, the gap between the walls and the tower, the porosity and the wall material were not varied. Intake cooling tower height of h was taken as the reference height with h = 8.5mm in 1/1000 scale model. The break walls were assumed to be rectangular with zero porosity. Table (1): Tested design parameters. Length of the walls (b) Angle relative to the radial direction (d) Gap between 2 walls (a) h/2 0 degree h/2 h 30 degree h 1.5h 45 degree 2h 3. Wind Tunnel Facility and Apparatus Experiments were conducted in the open wind tunnel in the Aerodynamic laboratory of the Faculty of Engineering at University of Technology, Sydney (UTS). c a f Plastic wall Wall Fig. (1): A Schematic diagram of curativedevices positioned on a plastic ring shape support. A schematic diagram of the test rig is presented in Fig. 2. The Open Wind Tunnel is approximately 10.5m overall length and has an octagonal test section measuring 610mm 610mm approximately 2m long. The range of air velocity in the tunnel ranges from approximately 0 to 60m/s. The induced draft fan it is a centrifugal type, which allows simulating the air exit of the tower in the real d b e

4 64 Mech. & Aerospace Eng. J., Vol. 3, No. 4, Winter 2008 Fan conditions. U-tube alcohol manometers, Pitot tubes, and pressure transducers were used to measure the required pressures and velocities in the tower and in the wind tunnel. Pressure transducers are continent devices for direct and automatic data acquisition by a computer. The arrangement of these devices has been calibrated using NACA0012 airfoil test case which its exact pressure distribution for low speed flows is available. The results of this test case showed that the error of this arrengemnt is less than 1.2%. The transducer outputs a linear voltage proportional to the applied pressure. A LabView virtual instrument (VI) software was developed to acquire data from the transducers. Centrifugal Induced Draught Fan Cooling Tower Model Fig. (2): Open wind tunnel. Wind Direction 3. Experimental Procedure The first step and the most delicate is to set the curative device and to well arrange it. The simulated stands for packing have to be at the level to the packing mesh representing the packing in the cooling tower. The Fig. 3 shows a positioned curative device. The arrangement of the device is shown clearly in Fig. 1. Once the device was installed, the data acquisition could begin The tests involved running the induced draft fan fixed at a frequency of 30Hz whilst varying the speed of the wind tunnel fan over the range from zero to maximum (60m/s) using the motor speed controllers. The wind tunnel fan was varied from 0 to maximum while recording measurements at regular intervals with the manometers. For each measurement, it is necessary to wait at least 1 minute to allow the stabilization of the alcohol in the manometers. Then this was repeated 3 times to evaluate the repeatability of the results. Turntable Curative device Cooling tower Fig. (3): Positioned curative device. 4. Analysis of Results Over twenty three curative devices were designed and manufactured using various design parameters shown in Fig. 1. Their effects on the pressure loss coefficient were quantified. In Fig s. 4-a and 4-b the pressure loss of the devices (Cp) is compared with the pressure loss of no-device case (Cp 0 ). Fig s. 5-a and 5-b show variation of DCp versus. Negative DCp means negative performance and positive DCp means that the pressure loss decreases as the curative device is used. The trend of the curves in Fig s. 5-a and 5-b are the same. The performance of the devices is negative at low wind velocity. As the wind velocity increases the presformance increases to positive value, reaches to its maximum and then decreases to zero or negative value. Consequently none of the curative device can be effective at nearly zero wind velocity and also at high wind velocity. The trend of the variation of the performance with with wind velocity is justifiable according to characteristics of the flow around the tower: When the wind velocity is nearly zero, the viscous effects are dominated, and the presence of the devices leads to more resistace (friction) force. Consequently the Cp will be greater than Cp 0. This can be seen in Fig s. 5-a and 5-b. Increasing the wind velocity causes non uniform flow around the tower for no device case as shown in Fig. 6-a. The pressure loss will increases due to this non uniform flow. For such conditions using the curative device changes the velocity distribution and consequently the pressure loss may be lesser than that of the no device case. For higher wind velocities flow separation occurs (Fig. 6-b) that raises the pressure loss for no device case. For such conditions using curative devises intensifies recirculation flows and the pressure loss gets higher. This can be seen in Fig. 7.

5 Effect of Wind 65 Cp Cp No device Device 32 Device 26 Device No device Device 19 Device 17 Device 18 Device 29 Device 31 (a) (b) Fig. (4): Pressure loss at the intake of the tower versus velocity ratio, (a)-cases with bad performance and (b)-cases with good performance. DCp DCP (a) Device 32 Device 26 Device (b) Device 19 Device 17 Device 18 Device 29 Device 31 Fig. (5): Percentage of performance improvement for devices, (a)-cases with bad performance and (b)-cases with good performance. Fig. (6): Flow around the tower at low speed flows. Cp No device Device 29 Device 17 Device 19 Device 22 Device 32 Device 26 Device 28 Device 31 Device 20 Device 27 Fig. (7): Pressure loss at the intake at high wind velocities. Among the tested devices, three of them (device 27, 19, 29) have better performance as shown in Fig s. 4 and 5. The specifications of these devices are given in in table 2. The maximum of efficiency is around 54% that can be attained using device 19 at around 1. It is noticed that the cooling efficiency is not on all the range but this maximum is located around the value of the velocity ratio obtained in the real situation [7]. The common characteristic of these devices is the axial inclination. Its value is 30 degree. It should be noted that only the length of the walls, the angle relative to the radial direction and the gap between 2 walls was tested. It is deduced that: The ideal angle relative to the radial is 30 degree. For this angle, the cooling rate is the best [7] consequently the pressure loss experiments carried out at this angle, It is difficult to deduce the optimal value of the length of the walls because the best devices of these comparisons have not the same length of the walls, It is difficult to deduce the optimal value of gap between 2 walls because the best devices of these comparisons have not the same gap between 2 walls, and

6 66 Mech. & Aerospace Eng. J., Vol. 3, No. 4, Winter 2008 The concept of many walls around the tower is the best one. Table (2): The 3 best devices. Device number Criteria Device 29 Device 19 Device 17 Geometry type Rectangular Rectangular Rectangular Number of walls Dimensions Height (f)* Length (b)* h h h 1.5h 1.5h h Radial angle (d)* Gap between 2 walls (a)* h 2h 2h * These dimensions are schematized on Fig Conclusion As it is shown, the maximum of efficiency is around 54%. This value is obtained with a dry cooling tower. At long term, this study applies to a wet coolin tower. It is why the obtained cooling effecincy can be less than the cooling efficiency with a wet cooling tower. Complementary tests are required to determine the optimal value for each characteristic. At present, only the optimal value for the radial angle is known: 30 degree which is determined based on maximum cooling rateb [7]. Other characteristics could be tested as the vertical angle, the gap between the wall and the tower, the height of the walls, the porosity and the type of material. It is known that the wind also affect the thermal characteristics of the natural draft cooling towers. Arrangmenof experimental model to study the effects of the towers is more complicated than the presented model. The future step of the others is to develop and experimental thermal model and dealing optimum arrrangment of the curative devices to compenents the reduction oe the towers thermal performance. Acknowledgments This research was funded by an ARC-Linkage grant supported jointly by Australian Research Council, and Delta Electricity. All these tests were peryomed in a wind tunnel on a scaled model. References 1. Zhenguo, Z. and Jinling, S., "The Unfavorable Effect of Natural Wind on Natural Draft Dry Cooling Towers and its Improvement Measures", China Institute of Water Resources and Hydropower Research, IWHR, Bender, T.J., Bergstrom D.J., and Rezkallah, K. S., "A Study on the Effects of Wind on the Air Intake Flow Rate of a Cooling Tower, Part 2., Wind Wall Study", J. Wind Eng. and Industrial Aerodynamics, Vol. 64, No. 1, pp , Du-Preez, A.F., "The Influence of Cross-winds on the Performance of Natural Draft Drycooling Towers", Mech. Eng. Dep t., Univ. of Stellenbosch, Stellenbosch, p. 7600, RSA, Gandemer, J.,"The Aerodynamic Characteristics of Windbreaks, Resulting in Empirical Design Rules", J. Wind Eng. and Industrial Aerodynamics, Vol. 7, No. 2, pp , Madadnia, J., Reizes, J., Behnia, M., Coombes, P., Koosha, H., Bojnordi, M., Al-Wakeed, R., and Hill, W., Wind Effects on the Operation of a Natural Draft Wet Cooling Tower, a) Performance Analysis", The 12th IAHR Symp. in Cooling Tower and Heat Exchangers, UTS, Sydney, Australia, Madadnia, J., Bojnordi, M., Koosha, H., and Reizes, J., "Wind Effects on Performance of a Natural Drought Wet Cooling Tower, b) Influence of the Packing Design (Model Tests)", 12th IAHR Symp. in Cooling Tower and Heat Exchangers, UTS, Sydney, Australia, Madadnia, J., Bojnordi, M., and Koosha, H., "Wind Effects on the Operation of a Natural Drought Wet Cooling Tower, c) Effectiveness Analysis of Performance Improving Devices (Scaled Model)", The 12th IAHR Symp. in Cooling Tower and Heat Exchangers, UTS, Sydney, Australia, Koosha, H., Madadnia, J., and Simoff, S., "Development of a Remote Access Research Site in Power Station Cooling Towers", The 12th IAHR Symp. in Cooling Tower and Heat Exchangers", UTS, Sydney, Australia, Samali, B. and Madadnia, J., Wind Simulation in an Environmental Wind Tunnel for Both Structure and Performance Studies, The 12th IAHR Symp. in Cooling Tower and Heat Exchangers, UTS, Sydney, Australia, Madadnia, J., Koosha, H., Bojnordi, M., and Al- Wakeed, R., An Overview of Experimental and Numerical Studies In Power Station Cooling Towers, The 12th IAHR Symp. in Cooling Tower and Heat Exchangers, UTS, Sydney, Australia, Madadnia, J., Behnia, M., and Al-Wakeed, R., Numerical Modelling and Validation of Natural Draught Cooling Towers under Crosswind, The 12th IAHR Symp. in Cooling Tower and Heat Exchangers, UTS, Sydney, Australia, Amur, G.Q., Madadnia J., Milton, B., Reizes J., Beecham, S., and Koosha, H., Study of Wind Effects on the Natural Draft Cooling Towers of

7 Effect of Wind 67 Mount Piper Power Station in NSW Australia Full scale Measurements, The Int. Conf. on Electric Supply Industry in Transition, Issues and Prospects for Asia, Bangkok Thailand, Amur, G.Q., Madadnia J., and Milton, B, Study of Wind Effects on the Natural Draft Cooling Towers of Mount Piper Power Station in NSW Australia, Wind Tunnel Study, The Int. Conf. on Electric Supply Industry in Transition, Issues and Prospects for Asia, Bangkok Thailand, Amur, G.Q., Madadnia, J., Milton, B., Reizes, J., and Koosha, H., Role of Plant Buildings in a Power Station Acting as a Barrier to the Wind Affecting the Natural Draft Cooling Tower Performance The 15th Australasian Fluid Mechanics Conf., Univ. of Sydney, Sydney, Australia, 2004.

8 68 Mech. & Aerospace Eng. J., Vol. 3, No. 4, Winter 2008